Hot-rolled steel strip provided for producing non grain-oriented electrical sheet, and method for the production thereof

ABSTRACT

The present invention relates to a hot-rolled steel strip for further processing to form non-grain oriented electrical sheet with the following composition (in % by weight) C: &lt;0.02%, Mn: ≦1.2%, Si: 0.1-4.4%, Al 0.1-4.4%, wherein the sum formed from the Si content and twice the Al content is &lt;5%, P: &lt;0.15%, Sn: ≦0.20%, Sb: ≦0.20%, the remainder iron and unavoidable impurities, with a strip thickness which is at most 1.8 mm, and with a partially softened structure which is characterised by a high intensity for the α fibre (fibre representation of orientation distribution functions) in the region of 0° to 60°, wherein the ratio I 112 /I 001  formed from the intensity I 112  of the position (112)&lt;110&gt; to the intensity I 001  of the position (001)&lt;110&gt; is &gt;0.4 and the ratio I 111 /I 001  formed from the intensity I 111  of the position (111)&lt;110&gt; to the intensity I 001  of the position (001)&lt;110&gt; is &gt;0.2.

The invention relates to a hot-rolled steel strip intended for theproduction of electrical sheet and a method for the production thereof.

In this context, the term “non-grain oriented electrical sheet” is takento mean a steel sheet or a steel strip which regardless of its texturecomes under the sheets mentioned in DIN 46 400 Part 1 or 4 and the lossanisotropy of which does not exceed the maximum values established inDIN 46 400 Part 1. The terms “sheet” and “strip” are used synonymouslyhere.

The production of non-grain oriented electrical sheet (NO electricalsheet) conventionally comprises the steps:

melting the steel,

casting the steel to form slabs,

if necessary, reheating the slabs,

using the slabs in a hot-rolling line,

pre-rolling the slabs,

finishing hot-rolling of the slabs to form a hot strip, of which the endthickness is between 1.8 mm and 5 mm, typically between 2 mm and 3 mm,

annealing and pickling of the hot strip wherein these hot striptreatments can be carried out as combined annealing pickling,

cold-rolling to an end thickness in the region of 0.75 mm to 0.35 mm orsmaller or, if necessary, cold-rolling of the hot strip to end thicknesstaking place in multi-stages with interposed annealing, and finalannealing of cold strips of this type in end thickness, which have beencold-rolled with a degree of total deformation of at least 65%, orannealing and subsequent rerolling with a degree of total deformation ofat most 20%.

Not until the cold-rolling process is softening of the structureachieved by a recrystallisation, degrees of total deformation of >65%being required to achieve the conventional end thicknesses of the endproduct “cold-rolled NO electrical strip” (starting point hot strip >1.8mm, end thickness 0.35 to 0.75 mm). Characteristic of a softenedstructure is an intensity distribution of the a fibre texture such thatan increased intensity of the component {112}<110>occurs and thecold-rolling component {001}<110> is largely removed.

Therefore the cold-rolling with these high degrees of total deformationcreates the prerequisite for the possibility of using a final annealing,which is conventional nowadays, in the form of a “short-time annealing”(through-type furnace—short times of high temperatures for the strip)with the aim of achieving a softened structure and an optimum grain sizein the finished product “cold-rolled NO electrical strip”.

The large number of working steps to be carried out in a conventionalprocedure of this type leads to high expenditure in terms of apparatusand costs. Therefore recently increased efforts have been made to designthe casting of the steel and subsequent rolling processes in the hotstrip production such that a hot strip with a thickness of ≦1.8 mm isproduced. One way to achieve this aim is a continuous sequence of thecasting and rolling process dispensing with the reheating and thepre-rolling.

For this purpose, so-called “casting/rolling plants” have been developedand set up. In these devices also known as “CSP plants”, the steel iscast to form a continuously drawn billet (thin slab) which is thenhot-rolled “in-line” to form hot strip. The experiences obtained inoperating casting/rolling plants and the advantages of thecasting/rolling carried out “in-line” have been documented, for examplein W. Bald et al. “Innovative Technologie zur Banderzeugung”, Stahl undEisen 119 (1999) No. 3, pages 77 to 85, or C. Hendricks et al.“Inbetriebnahme und erste Ergebnisse der Gieβwalzanlage der ThyssenKrupp Stahl AG”, Stahl und Eisen 120 (2000) No. 2, pages 61 to 68.

However, even in the framework of conventional plant engineering forhot-rolling, including the pre- and intermediate rolling, an attempt ismade in the use of conventional slabs to achieve hot strip thicknessesof ≦1.5 mm, see for example JP 2001 123225 A2.

The invention was based on the object of realising an economicallyproducible hot strip with a partially softened structure with athickness of at most 1.8 mm which, owing to these properties, isespecially suitable for producing high-grade electrical sheets.

This object is achieved starting from the above-described prior art by ahot-rolled steel strip, which has the following composition (in % byweight):

C: <0.02%

Mn: ≦1.2%

Si: 0.1-4.4%

Al 0.1-4.4%,

wherein the sum formed from the Si content and twice the Al content ([%Si]+2×[% Al]) is <5%,

P: <0.15%

Sn: ≦0.20%

Sb: ≦0.20%,

the remainder iron and unavoidable impurities,

the strip thickness of which steel strip is at most 1.8 mm, and

which has a partially softened structure, which is characterised by ahigh intensity of the α fibre (fibre representation of orientationdistribution functions) in the range to 60°.

The invention proceeds from the recognition that with a choice of asuitable method of production, a hot strip can be provided which alreadyin the hot-rolled state has a structure which can be produced only bycold-rolling with high degrees of deformation in a conventional mannerof production. Thus, a hot strip composed and made up according to theinvention with a strip thickness of at most 1.8 mm has a partiallysoftened structure. This structure is distinguished by high intensitiesof the α fibre in the range of angles up to 60° for specific positions,in other words in an angle range in which, in the case of conventionalhot strips of comparable composition, no noteworthy intensities would begenerally able to be established for these positions. The highintensities of the specific positions (112)<110> and (111) <110> arecharacteristic, wherein for the ratios of the intensities I₁₁₂ of theposition (112)<110> to the intensity I₀₀₁ of the position (001)<110> avalue >0.4 is produced and for the ratios of the intensity I₁₁₁ of theposition (111)<110> to the intensity I₀₀₁ of the position (001)<110> avalue >0.2 is produced. Owing to this composition, hot strip accordingto the invention can be processed in an excellent manner to formcold-rolled NO electrical sheet, the end thickness of which is typically0.35 mm to 0.75 mm, in particular 0.2 mm, 0.35 mm, 0.50 mm or 0.65 mm.

Conventional hot strips differ from those according to the invention inthat in the case of these noteworthy intensities only occur in the rangeof up to 25° (−30°), while for the components (112)<110> and (111)<110>no further higher intensities can be established. In the conventionalhot strips, an intensity maximum of the α fibre structure is typicallypresent at 0°, from which the intensity decreases with an increasingangle. This intensity distribution of the α fibre corresponds to ahardened structure. Only owing to the cold-rolling process is asoftening of the structure achieved in the case of such steel strips bya recrystallisation in the subsequent annealing. For this purpose,degrees of total deformation of more than 65% are required which, on theone hand, assume a specific minimum thickness of the hot strip to becold-rolled and, on the other hand, a considerable rolling power in thecold-deformation of the strip.

Hot strip according to the invention is composed in comparison such thatthe intensities of the component (112) <110> and the intensities of theposition (111)<110> are high. At the same time, hot strip according tothe invention has a particularly small end thickness. The hot stripaccording to the invention thus creates far more favourable conditionsfor the subsequent processing than the conventional hot strips canachieve. Thus, hot strip according to the invention starting from itssmall thickness of at most 1.8 mm with a minimised total deformation canbe cold-formed into a non-grain oriented electrical sheet, theproperties of which are at least equal to the properties ofconventionally produced NO electrical sheets.

In relation to the terms used α fibre, intensity and position, it shouldbe remembered that the texture of a crystalline phase is describedquantitatively by means of the orientation distribution function.

The orientation distribution function describes the relative position ofthe crystal coordinate system and sample coordinate system. Theorientation distribution function allocates each point in the space anorientation density or intensity. As representation of the orientationdistribution function is very complicated and not very graphic, asimplified description is selected with the aid of fibres. The fibresrelevant for steels are:

α fibre, γ fibre, η fibre, ζ fibre, δ fibre.

In the α fibre observed here, the <110> direction is parallel to therolling direction; it extends between the positions (001)<110> and(110)<110>.

Hot strip according to the invention has a particular favourablesoftening state for further processing when its strip thickness is atmost 1.2 mm. With hot strip according to the invention which is as thinas this, the ratio I₁₁₂/I₀₀₁ formed from the intensity I₁₁₂ of theposition (112) <110> to the intensity I₀₀₁ of the position (001)<110> ofthe α fibre is >0.75 and the ratio I₁₁₁/I₀₀₁ formed from the intensityI₁₁₁ of the position (111)<110> to the intensity I₀₀₁ of the position(001)<110> of the α fibre is >0.4. Hot strip softened in this way can beprocessed with particularly small degrees of deformation into NOelectrical sheet.

Hot strips according to the invention with hot strip thicknesses of ≦1.8mm can be manufactured in various ways; conventional hot strip lineswith possibilities for producing the above thicknesses, continuouscasting and rolling plants (casting of thin slabs with subsequentin-line hot-rolling), thin strip casting plants with subsequent singleor multi-stage hot-rolling of the thin strip.

According to an advantageous configuration of the method according tothe invention, at least one pass of the hot-rolling is carried out attemperature, at which the hot strip has an austenitic structure, and aplurality of subsequent passes of the hot-rolling are carried out attemperatures in which the hot strip has a ferritic structure. Owing torolling deliberately carried out in this way in the individual phasestate regions, in particular in the case of converting alloys, hotstrips can be produced which have optimised properties with respect tothe demands placed on NO electrical sheets. It has been shown, forexample, that owing to a suitable combination of the phase sequence inhot-rolling in conjunction with specific end rolling and coilertemperatures, a decisive raising of the magnetic polarisation can beachieved. To ensure that at least the last pass of the hot-rolling iscarried out with a ferritic structure in the hot strip, the end rollingtemperature during hot-rolling should be less than 850° C.

During the hot-rolling, at least during one of the last deformationpasses, rolling is carried out with lubrication. Owing to thehot-rolling with lubrication, on the one hand, smaller sheardeformations occur, so that the rolled strip receives a more homogeneousstructure over the cross-section as a result. On the other hand, owingto the lubrication, the rolling forces are reduced, so that a greaterreduction in thickness is possible over the respective rolling pass.Therefore, according to the desired properties of the electrical sheetto be produced, it may be advantageous if all the forming passes takingplace in the ferrite region are carried out with a rolling lubrication.

Hot strips according to the invention, can be produced in particularwith reliably reproducible working results in that initially a steelcomposed according to the invention is melted and then the steel is castinto thin slabs which are then continuously (“in-line”) hot-rolled toform hot strip. The extent of total deformation achieved during thehot-rolling is preferably at least 90%, the hot-rolling generally beingcarried out in a plurality of passes.

The continuous sequence particular to known continuous casting androlling, of casting the steel to form thin slabs and hot-rolling thethin slabs to form a hot strip, also allows working steps to bedispensed with in the production of hot strips according to theinvention, as for example the reheating of the slabs and pre-rolling.Moreover, it has been shown that dispensing with the according workingsteps influences the material state in the various production phases.This sometimes differs considerably from that achieved in theconventional production of hot strip in which at the beginning thecooled slab is reheated. In particular it is the macroliquations and thesolution and precipitation state which differentiates hot stripsproduced according to the invention from those produced conventionally.In addition, the forming process during the hot-rolling takes placeduring continuous in-line casting and rolling in favourable thermalconditions. Thus, the rolling passes can be applied with higher degreesof deformation and the deformation conditions can be used in a targetedmanner to control the structure development.

In the use of continuous casting and rolling in the hot strip accordingto the invention, the phosphorous content is preferably limited to lessthan 0.08% by weight in order to achieve adequate casting properties.

The invention will be described hereinafter with the aid of embodiments.In the graph, the curve of the orientation distribution function(orientation density) is plotted for three examples over the angle Φ.“Φ” is one of the eulerian angles which describe the relative positionof the crystal coordination and sample coordination system. Specialpositions are simultaneously plotted: (001)<110> , (112)<110>,(111)<110> and others. To determine the properties of an example for ahot strip Wb_(E) according to the invention and two comparison examplesfor hot strips Wb_(v1) and Wb_(v2) not according to the invention, asteel with (in % by weight or ppm by weight)<30 ppm C, 0.2% Mn, 0.050%P, 1.3% Si, 0.12% Al, 0.01% Si and as the remainder Fe and impuritieswas melted.

In the case of the hot strip Wb_(v1) manufactured for comparison, themelted steel is cast to form a slab, which is then cooled in aconventional manner, reheated, pre-rolled and hot-rolled to an endthickness of 2.5 mm. The hot strip Wb_(v1) thus obtained, for anorientation angle Φ of 0° to 20°, had an orientation thickness of the αfibre determined in the strip centre, of at least 4, while theorientation thickness for angles Φ of more than 20° was regularly lessthan 3. The value of the ratio I₁₁₂/I₀₀₁ of the intensity I₁₁₂ of theposition (112)<110> to the intensity I₁₁₀ of the position (001)<110> ofthe α fibre was accordingly likewise below 0.1 like the value of theratio I₁₁₁/I₀₀₁ of the intensity I₁₁₁ of the component (111)<110> to theintensity I₁₁₀ of the component (001)<110>.

The curve of the orientation density over the angle Φ is shown in thegraph for the hot strip Wb_(v1) serving for comparison as a dotted line.

The high density in the region of small angles and the low density inthe region of large angles prove that the hot strip Wb_(v1) was in ahardened state in which it firstly has to be subjected to an expensivecold-rolling and after-treatment in order to be able to be used as a NOelectrical sheet.

In order to produce the hot strip Wb_(v2) also manufactured forcomparison, the same steel is firstly cast in a continuous casting androlling plant to form a thin slab which was then hot-rolled also“in-line” in a plurality of passes to a hot strip end thickness of 3 mm.

The hot strip Wb_(v2) thus obtained, for an orientation angle Φ of 0° to20°, like the hot strip Wb_(v1), had an orientation density of the αfibre determined in the strip centre of at least 4, while theorientation density for angles Φ of more than 20° was regularlysignificantly less than 3. The value of the ratio I₁₁₂/I₀₀₁ of theintensity I₁₁₂ of the position (112)<110> to the intensity I₁₁₀ of theposition (001)<110> of the α fibre was 0.2, while the value of the ratioI₁₁₁/I₀₀₁ of the intensity I₁₁₁ of the position (111)<110> to theintensity I₁₁₀ of the position (001)<110> only reached 0.06.

The curve of the orientation density over the angle Φ is shown in thegraph as a dash-dot line for the hot strip Wb_(v2) serving as acomparison.

In the case of the hot strip Wb_(v2) the high density in the region ofsmaller angles, and the low density in the region of large angles, alsoproves that the hot strip Wb_(v2) was in a hardened state in which itfirstly had to be subjected to an extensive cold-rolling andafter-treatment in order to be able to be used as NO electrical sheet.

The hot strip Wb_(E) according to the invention is also produced fromthe same steel as the hot strip Wb_(v1) manufactured for comparison. Forthis purpose, the relevant steel is also cast in a continuous castingand rolling plant to form a thin slab which is then also hot-rolled“in-line” in a plurality of passes. In contrast to the hot stripWb_(v2), the end thickness of the hot strip was only 1.04 mm, however.

The hot strip Wb_(E) thus obtained for all orientation angles Φ in therange of 0° to 60°, had an orientation density of the α fibre determinedin the strip centre of at least 4. The orientation density only droppedto below 3 in the angle range of more than 60°. The value of the ratioI₁₁₂/I₀₀₁ of the intensity I₁₁₂ of the position (112)<110> to theintensity I₁₁₀ of the component (001)<110> of the a fibre was at a highlevel, namely 0.81. In the same way, the value of the ratio I₁₁₁/I₀₀₁ ofthe intensity I₁₁₁ of the position (111)<110> to the intensity I₁₁₀ ofthe position (001)<110> reached a high level, namely 0.54.

The curve of the orientation density over the angle Φ is shown as asolid line in the graph for the hot strip Wb_(E) according to theinvention.

The high orientation densities up to an angle of 60° and the highintensities of the component (112)<110> and (111)<110> proves that thehot strip according to the invention is in a substantially partiallysoftened state.

1. Hot-rolled steel strip for further processing to form non-grainoriented electrical sheet with the following composition in % by weightC: <0.02% Mn: ≦1.2% Si: 0.1-4.4% Al: 0.1-4.4%; wherein the sum formedfrom the Si content and twice the Al content is <5%, P: <0.15% Sn:≦0.20% Sb: ≦0.20%, the remainder iron and unavoidable impurities, with astrip thickness which is at most 1.8 mm, and with a partially softenedstructure which is characterised by a high intensity of the a fibre(fibre representation of orientation distribution functions) in theregion of 0° to 60°, wherein the ration I₁₁₂/I₀₀₁ formed from theintensity I₁₁₂ of the position (112)<110> to the intensity I₀₀₁ of theposition (001)<110> is >0.4 and the ration I₁₁₁/I₀₀₁ formed from theintensity I₁₁₁ of the position (111)<110> to the intensity I₀₀₁ of theposition (001)<110> is >0.2.
 2. The hot strip to of claim 1, wherein thestrip thickness is at most 1.2 mm and the ratio I₁₁₂/I₀₀₁ formed fromthe intensity I₁₁₂ of the position (112)<110> to the intensity I₁₁₀ ofthe position (001)<110> of the a fibre is >0.75 and the ration I₁₁₁/I₀₀₁formed from the intensity I₁₁₁ of the position (111)<110> to theintensity loot of the position (001)<110> of the a fibre is >0.4.
 3. Thehot strip according to of claim 1 wherein said hot strip at a first hightemperature, has a ferritic structure, at a second temperature lyingbelow the first temperature, it has a ferritic/austenitic structure, ata third temperature lying below the second temperature, it has anaustenitic structure, at a fourth temperature lying below the thirdtemperature it has an austenitic/ferritic structure and at a fifthtemperature lying below the fourth temperature it again has a ferriticstructure.
 4. A method for producing hot strip composed according toclaim 1 comprising the following working steps: melting the steel,casting the steel to form thin slabs, continuous (“in line”) hot-rollingof the thin slabs following the casting of the steel.
 5. The method ofclaim 4, wherein the phosphorous content in the hot strip is <0.08% byweight.
 6. The method of claim 4, wherein the degree of totaldeformation achieved during the hot-rolling is at least 90%.
 7. Themethod for producing hot strip composed according to claim 1, comprisingthe following working steps: melting the steel, casting the steel toform a thin strip, continuous (“in-line”) hot-rolling following thecasting of the steel in one or more passes.
 8. The hot strip of claim 7,wherein its P content is <0.08% by weight.
 9. The method of claim 4further comprising carrying out the hot-rolling in a plurality ofpasses.
 10. The method of claim 9, further comprising carrying out atleast one pass of the hot-rolling at temperatures at which the hot striphas an austenitic structure, and carrying out a plurality of subsequentpasses of the hot-rolling at temperatures in which the hot strip has aferritic structure.
 11. The method of claim 4, wherein the end rollingtemperature during hot-rolling is less than 850° C.
 12. The method ofclaim 4, further comprising carrying out with lubrication at least thelast rolling pass during the rolling in the ferrite area.
 13. A methodfor producing non-grain oriented electrical sheet from a hot stripcomprising the following composition in % by weight: C: <0.02% Mn: <1.2%Si: 0.1-4.4% Al: 0.1-4.4%; wherein the sum formed from the Si contentand twice the Al content is <5%. P: <0.15% Sn: ≦0.20% Sb: ≦0.20%. theremainder iron and unavoidable impurities, with a strip thickness whichis at most 1.8 mm, and with a partially softened structure which ischaracterised by a high intensity of the α fibre (fibre representationof orientation distribution functions) in the region of 0° to 60°,wherein the ration I₁₁₂/I₀₀₁ formed from the intensity I₁₁₂ of theposition (112)<110> to the intensity I₀₀₁ of the position (001)<110>is >0.4 and the ration I₁₁₁/I₀₀₁ formed from the intensity I₁₁₁ of theposition (111)<110> to the intensity I₀₀₁ of the position (001)<110>is >0.2 and produced according to claim 4, further comprising thefollowing working steps: pickling or annealing and pickling of the hotstrip, cold-rolling of the hot strip intermediate annealing of the coldstrip final annealing or annealing with subsequent deformation with atotal degree of deformation of less than 20%.
 14. The method of claim13, further comprising carrying out the cold rolling in at least twostages with an intermediate annealing.